1. Field of the Invention
The invention is concerned with transparent overlays to protect documents from tampering and is especially concerned with such overlays which contain patterns and legends that are difficult to counterfeit and thus also function to authenticate the documents.
2. Description of the Related Art
Documents often have adherent transparent overlays to provide protection against dirt, moisture, and general wear and tear. A typical transparent overlay has a plastic film bearing an aggressive adhesive layer by which it can be permanently adhered to the face of a document. When the plastic film incorporates a message such as a design that does not obscure the underlying information, a transparent overlay can afford an additional degree of protection, especially when the message-containing plastic film is difficult to remove without being destroyed and also is difficult to counterfeit. For example, many credit cards presently are made to exhibit holographic images which may be transparent but often are opaque and thus confined to an area not bearing information.
Even when the plastic film of an overlay is made to be too fragile to be removed as a single piece or, if removed, so contorted that it cannot be reapplied, a nagging concern remains that a clever, deft person might be able to remove it without undue damage (e.g., by the use either of hot or exceedingly cold temperatures) and to alter the face of the document (e.g., by replacing a portrait or photograph that identifies the bearer). Even when doing so would be discernible under expert examination, ordinary use of most documents usually precludes such an examination except under extraordinary circumstances. For example, when the document is a passport, a customs official rarely is allowed more than a minute or two to check both the document and its bearer unless there is some external evidence to suggest a more careful examination.
A transparent overlay which can contain a pattern or legend that does not obscure underlying information is disclosed in U.S. Pat. No. 3,801,183 (Sevelin et al.). That pattern or legend is invisible or only faintly visible to the naked eye under diffuse light and becomes readily legible only when viewed retroreflectively. Such overlays are currently manufactured and sold as CONFIRM brand security film by the Minnesota Mining and Manufacturing Company.
Because the CONFIRM brand security film is fragile and has a layer of an aggressive adhesive by which it can be bonded permanently to a document, it may be impossible to peel the sheeting from the document and reapply it without leaving readily noticeable evidence of tampering. Nevertheless, some issuers of documents request even greater assurance against tampering.
Subsequent to the aforementioned U.S. Pat. No. 3,801,183, a number of patents have issued disclosing other transparent overlays, each of which can be imaged with a pattern or legend that is noticeable only when viewed retroreflectively and can be adhesively bonded to a document without obscuring the face of the document. See, for example, U.S. Pat. No. 4,099,838 (Cook et al.), the overlay of which has the additional feature of causing the color of the reflection in the background areas to be different from the color of the reflection in the image areas. See also U.S. Pat. Nos. 4,688,894 (Hockert) and 4,691,993 (Porter et al.), each of which discloses a transparent overlay that functions in the same way as that of the Sevelin patent while having the added capability of permitting an authenticating message to be formed in the overlay after it has been adhesively bonded to a document. However, none of the transparent overlays of those three patents offers better assurance against unnoticeable tampering than does the overlay of the aforementioned U.S. Pat. No. 3,801,183 or the CONFIRM brand security film.